1. Field of the Invention
The present invention generally relates to radio communication systems and, more particularly, to radio communication systems using spread spectrum techniques.
2. State of the Art
In radio communication systems, the term "spread spectrum" refers to techniques whereby radio signals that are assigned to narrow frequency bands (i.e., spectrums) are distributed over broader frequency bands. Consequently, spread spectrum broadcasts provide signals that have relatively wide frequency bands but relatively low power densities. One effect of frequency spreading is to reduce the power peaks in the operating frequency band and, thereby, to reduce the potential for interference with other nearby communication systems operating in the sam frequency band.
In conventional practice, frequency spreading is accomplished by multiplying a modulated signal by a second signal that comprises a spreading code. Accordingly, in spread spectrum broadcasting, only receivers that have knowledge of the spreading code being used by a particular transmitter will be able to intelligibly receive (i.e., decode) spread spectrum broadcasts from that transmitter. Stated conversely, a spread spectrum receiver will reject all broadcast signals that are not encoded with the spreading code which is used by the receiver.
In spread spectrum communication systems, the information transferred between transmitter/receiver units necessarily includes pattern synchronization information as well as data. The pattern synchronization information is needed for synchronizing receivers and transmitters to the spreading code to be used. Without synchronization, data could not be accurately recovered by a receiver.
It should be understood, however, that time must be spent by receivers to synchronize, or "lock-up," to the spreading code used by a transmitter. In practice, the greater the synchronization period, the lower the overall data throughput between a spread spectrum transmitter and receiver.